High ACT score, low SAT scores for selective colleges that require all test scores?

On my first SAT I received a 2100 and on my next a 2040 (I know it’s bad) but then I took the ACT with no prep and received a 35 composite score (34/35/35/36). I am applying to a few selective colleges that require all test scores to be sent in (ex. Stanford, uPenn). Should I still retake my SAT even if my ACT score is good?

No. The colleges will take the better of the SAT or ACT (Great score!). Just make sure you have take two SAT II subject tests if required/recommended for the colleges you wish to attend. Some might require ACT with writing.